This invention relates to an electrodialytic process for removal of SO.sub.2 from gases containing same and production of SO.sub.2 and NaOH. More particularly, the invention relates to the removal of SO.sub.2 from gases containing same by spray-drying or aqueous solution scrubbing with alkaline sodium sources, e.g., trona ore, nahcolite ore or soda ash, and the production of SO.sub.2 and NaOH from spent sodium materials (from SO.sub.2 scrubbing) using therefor electrodialytic water splitters.
Environmental considerations precludes the burning of fuels with high sulfur content, since the consequent production of sulfur dioxide presents a serious pollution problem. To overcome this, the sulfur in the fuel must be removed prior to, during or after combustion. For users of large quantities of fuel, such as utility companies, removal of sulfur after combustion generally has been recognized as most feasible. In this regard, considerable research has been done and a number of processes for removal of sulfur dioxide from the combustion gases have been developed. Among these are lime and limestone scrubbing processes, magnesium oxide scrubbing processes, sodium scrubbing processes with thermal regeneration (e.g., see U.S. Pat. Nos. 3,477,815 and 3,485,581), sodium scrubbing with electrolytic regeneration (e.g., see U.S. Pat. No. 3,475,122) citrate processes [U.S. Bureau of Mines (Report of Investigations 7774, (1973)], phosphate processes [Stauffer Chemical, Chem. Eng. 81, July 8, pp. 46-47 (1974)], double alkali processes and the catalytic oxidation process for making sulfuric acid. A review of these and other processes has been made by A. V. Slack of Noyes Data Corp. ["Sulfur Dioxide Removal from Waste Gases"--Pollution Control Review No. 4 (1971)].
Many considerations enter into the decision of which type of process might be best. Among these are capital and operating costs, reliability, flexibility and operation, production of useful sulfur products, production, and disposal of byproducts. For example, sodium-based throwaway systems for flue gas desulfurization are disclosed in Power, October 1982, Special Report, pages S-10 to S-17. However, these throwaway systems often produce a liquid waste which requires a large, carefully contained holding pond to evaporate water from and concentrate the species dissolved in the liquid waste. Accordingly, the use of sodium-based throwaway systems are limited to regions where large tracts of land are available for ponding, the evaporation rate is high and exceeds rainfall, and low sulfor coal (containing about 1% by weight sulfur) is burned to produce less waste to be disposed of.
U.S. Pat. Nos. 4,082,835 and 4,107,015 disclose electrodialytic methods of removing and recovering SO.sub.2 from flue gases containing same by (1) scrubbing the flue gas with an aqueous alkaline solution, e.g., NaOH, Na.sub.2 SO.sub.3, a combination of both or any other relatively basic solution; (2) dividing the treated scrubbing solution into two streams; (3) feeding the two streams into a two-component electrodialytic water splitter; (4) withdrawing from said electrodialytic water splitter an aqueous alkaline solution which is recycled to the scrubbing step, and a SO.sub.2 -containing solution; (5) stripping the SO.sub.2 from the solution containing same to produce a solution containing waste sulfate of which a portion is purged and (6) recycling the remainder of the waste sulfate containing solution to the scrubbing step. Thus, U.S. Pat. Nos. 4,082,835 and 4,107,015 disclose closed loop processes wherein the scrubbing solution is substantially completely recycled.
In addition, there are several common sources of SO.sub.2 -containing gas streams in which SO.sub.2 is too dilute to be economically utilized directly as a source of sulfur for H.sub.2 SO.sub.4 or sulfur manufacture. However, these common sources of SO.sub.2 are of sufficient volume or concentration to present an environmental hazard if they are vented to the atmosphere. Major sources of such streams are: lead smelters, which might emit steams with SO.sub.2 concentrations of about 0.1 to 5 percent; sulfuric plants emitting streams containing about 0.05 to about 1 percent SO.sub.2 ; and flue gases from oil or coal fired boilers which may contain from about 0.004 percent to about 0.5 percent SO.sub.2.
Among the methods used for treating such gases is scrubbing of the stream or sodium bases. In some cases, e.g., the dual alkali process or the processes of U.S. Pat. Nos. 4,082,835 or 4,107,015, are employed. In other cases, an inexpensive source of sodium base, such as naturally occurring mineral trona or nacholite or other Na.sub.2 CO.sub.3 /NaHCO.sub.3 mixtures, is used for scrubbing and the spent scrubbing material is merely disposed of. Disposal of the spent scrubbing material can represent a substantial operating cost because of the environmental problems associated with leaching of soluble minerals therefrom and subsequent contamination of groundwater supplies.
One such throw-away process is generated by the aqueous scrubbing process known as solution scrubbing. In the process of solution scrubbing, a solution of a sodium base such as trona, nacoholite, soda ash, calcined trona or calcined nacoholite, or a liquors produced during the processing of trona ore, soda ash or nacoholite, or some other sodium carbonate and/or NaHCO.sub.3 solutions is introduced into a scrubber for contact with a gas containing SO.sub.2. The scrubber may be a packed tower, a tray absorption tower, or other device known to one skilled in the art. In a tower, a substantial amount of SO.sub.2 is removed from the incoming gas and the gas exiting the scrubber contains a small enough amount of SO.sub.2 so as not to pose an environmental hazard. The solution stream produced by such a process will comprise, and normally consists essentially of, NaHCO.sub.3, NaHSO.sub.3, Na.sub.2 SO.sub.4, and Na.sub.2 SO.sub.3. Typically, the solution will contain from about 0.1 to 15 percent NaHSO.sub.3, about 0.1 to about 15 percent NaSO.sub.4 and about 0.1 to about 10 percent Na.sub.2 SO.sub.3 (all weight percent). The stream constitutes the waste which must be disposed of, normally by ponding.
Another throw-away waste is generated by the scrubbing process known as spray drying. A solution or slurry of sodium base such as trona, nacoholite, soda ash, calcined trona or calcined nacoholite, or a liquor produced during processing of trona ore, soda ash, or nacoholite, or other sodium carbonate and/or sodium bicarbonate solution is sprayed into a tower wherein the heat from the SO.sub.2 -containing gas causes the water to evaporate from the solution or slurry. The SO.sub.2 in the incoming gas reacts with the base either in the spray or the dry particle form. In the process of spray drying, substantial quantities of SO.sub.2 (normally about 40 percent up to about 99 percent of the SO.sub.2), are removed from the gas resulting in gaseous effluent depleted in SO.sub.2. The dry waste comprises Na.sub.2 SO.sub.4, Na.sub.2 SO.sub.3, sometimes NaHSO.sub.3, as well as unreacted Na.sub.2 CO.sub.3 and/or NaHCO.sub.3. Normally, only small amounts of sodium bisulfite are present in the reaction mixture recovered when spray-dryer absorption is used.